The outbox sweep — taken, scoped (2026-08-29)

A design note from the engineering record, moved out of ROADMAP.md on 2026-09-03 and kept as written. The roadmap itself now holds only the project's state; the reasoning lives here.

The second lever the inversion entry named. Every pass swept all 1,024 slots for a streaming one to drain, and the miss path — two flag loads per slot, none set — measured 1.2–1.3 µs per pass in isolation, on a pass that carries one or two requests at low concurrency. The ceiling, with the sweep skipped outright (bench/results/outbox-sweep/, comparators within the clean band on every arm): hello +3.5% at c16 and +2% at c64, the inverted executor +4.1% at c16 — and the pump −2.9% rps at +6% CPU at c16, ±0 at c256. That last row is the finding: under the pump the microsecond was accidental pacing. A loop thread that returns to wait a microsecond sooner finds fewer events, batches fewer submits, and the executor thread takes more wakes per request; at c256 the batches are large regardless and it makes no difference either way.

So the gate is scoped. OffloadLoopState.streaming_hint is an upper bound on the flagged slots — raised by the two sites that set a stream flag (both in _finish_response), recounted by the sweep itself, never touched by the many sites that clear one, so an under-count (a stream nothing drains) cannot happen and an over-count costs one sweep — and the sweep runs only while it is non-zero. EXCEPT when the pool says sweeps_every_pass, which the pump wiring sets and nothing else does: its loop keeps the per-pass sweep, and the reason is written on the field. Realized (same day, comparators clean): hello 152.3k → 157.4k at c16 (+3.3%, +5.5% per core), +1.3% at c64; the inverted executor 59.3k → 62.0k (+4.6%), 69.6k/core; the pump 60.0k → 59.7k @0.98, parity within noise, as intended. Guards: the streaming smokes, sabotaged three ways — never sweep (counter and ws fail), the SSE site not raising the hint (counter fails, ws passes), the WS site not raising it (ws fails, counter passes).